Electrical, thin film termination

ABSTRACT

An apparatus for operably connecting an electrical source to a conductive coating or film. The apparatus may include a substrate made of a structural material. A conductive coating or thin film may be applied to the substrate. An interface layer may be applied over the conductive coating and conduct electricity thereto while transferring insufficient force to separate the conductive coating from the substrate. A conductor, for providing electricity to the interface layer comprising strands configured to be separable and electrically conductive, may be positioned in contact with the interface layer. A clamping mechanism may apply a clamping load urging the conductor toward the conductive coating. The strands of the conductor may be formed to distribute mechanical stress and strain induced by thermal expansion and the clamping load sufficiently to substantially reduce damage to the mechanical and electrical integrity of the conductive coating.

RELATED APPLICATIONS

This Patent Application is a continuation-in-part of U.S. patentapplication Ser. No. 09/738,724 filed on Dec. 15, 2000 entitled DURABLE,NON-REACTIVE, RESISTIVE FILM HEATER.

BACKGROUND

1. The Field of the Invention

This invention relates to electrical terminations and, moreparticularly, to novel systems and methods for transferring electricalcurrent from a lead to a thin film.

2. The Background Art

The semiconductor manufacturing industry relies on numerous processes.Many of these processes require transportation and heating of de-ionized(DI) water, acids, and other chemicals. By clean or ultra-pure is meantthat gases or liquids cannot leach into, enter, or leave a conduitsystem to produce contaminants above permissible levels. Whereas otherindustries may require purities on the order of parts-per-million, thesemiconductor industry may require purities on the order ofparts-per-trillion.

Chemically clean environments for handling pure de-ionized (DI) water,acids, chemicals, and the like, must be maintained free fromcontamination. Contamination in a process fluid batch may destroyhundreds of thousands of dollars worth of product. Several difficultiesexist in current systems for heating, pumping, and carrying processfluids (e.g., acids, DI water). Leakage into or out of a process fluidconduit must be eliminated. Moreover, leaching and chemical reactionbetween any contained fluid and the carrying conduits must beeliminated.

Elevated temperatures in semiconductor processing are often over 100 C.,and often sustainable over 120 C. In certain instances, temperatures ashigh as 180 C. may be approached. It is preferred that all process fluidheating and carrying mechanisms virtually remove the possibility ofcontact with any metals, regardless of the ostensibly non-reactivenatures of such metals. It is desirable to prevent process fluidcontamination, even in the event of a catastrophic failure of anyelement of a heating, transfer, or conduit system.

Conventional immersion heaters place a heating element, typicallysheathed in a coating, directly into the process fluid. The heatingelement and process fluid are then contained within a conduit.Temperature transients in immersion heaters may overheat a sheath up toa melting (failure) point. A failure of a sheath may directly result inmetallic or other contamination of the process fluid. Meanwhile,temperature transients in radiant heaters may fracture a rigid conduit.

A heating alternative is needed that does not have the risks associatedwith conventional radiant and immersion-heating elements. A system isneeded that is both durable and responsive for heating process fluids.Failure that may result in fluid contamination is an unacceptable risk.

BRIEF SUMMARY AND OBJECTS OF THE INVENTION

It is a primary object of the present invention to provide a heater forhandling process fluids at elevated temperatures in the range of 0 C. to180 C. It is an object of the invention to provide a heater havingelectrical resistance in close proximity to a process fluid for heatingby conduction and convection without exposing process fluids tocontamination, even if electrical failures or melting of conductivepaths should occur within a heater.

Consistent with the foregoing objects, and in accordance with theinvention as embodied and broadly described herein, a method andapparatus are disclosed in one embodiment of the present invention asincluding a heater comprising a substrate. The substrate may be formed amaterial having suitable strength, heat transfer characteristics,non-reactivity, and coating adherence. The substrate may function toseparate a heating element from the fluid to be heated. The substratemay have any suitable shape which may promote efficient heat transfer tothe fluid passing there across. In certain embodiment, the substrate maybe formed as a conduit to transfer the fluid.

In one embodiment, the substrate is one or more tubes of quartz. In suchan embodiment, the tubes may be abutted end-to-end with an adapter (e.g.fluorocarbon fitting) fitted to transfer the fluids between two tubes ina series. One pass or passage, comprising one or more tubes of quartz ina series, may be fitted on each end to a manifold (e.g. header/footer)comprised of a fluorocarbon material properly sealed for passing liquidinto and out of the individual passage.

Individual tubes or conduits may improve the temperature distributiontherein by altering the internal boundary layer of heated fluids passingtherethrough. In one embodiment, a baffle tube, within the outer tube,may have a plug serving to center the baffle in the heating tube. Theplug may restrict flow, such that the fluid inside the baffle does notchange dramatically. Thus an annular flow between the baffle tube andthe outer heating tube may maintain a high Reynolds number in the flow,enhancing the Nusselt number, heat transfer coefficient and so forth.Moreover, the temperature distribution may be rendered nearer to aconstant value across the annulus, rather than running with a cold,laminar core.

In one embodiment, a heater may be manufactured by depositing, plating,or otherwise adhering a resistive coating or layer to a surface of thesubstrate. The resistive coating may be any material having a properbalance of conductivity, resistivity, and adherence. In certainembodiments, the substrate surface may be roughened or otherwiseprepared to promote adherence of the resistive coating thereto. In oneembodiment, electroless nickel may be plated on a roughened (textured)surface of the substrate.

A resistive, conductive coating may extend along any selected length ofthe substrate. The resistive coating may be configured to connect inseries or to multi-phase power along the length of a single substrate.In one embodiment, a quartz tube may be roughened, etched, dipped,coated, and protectively coated. The quartz tube need not be heated tosinter the conductive layer. The conductive coating may be plated as acontinuous ribbon of well-adhered, resistive, conducting, metallicmaterial.

The electrical length of the heated portion (i.e. the area coated withthe resistive coating) may be adjusted by application of an end coatingfor distributing current. Electrical current may be applied to the endthe coating or directly to the resistive coating by any suitabletermination. In selected embodiments, a electrical lead may be solderedto directly to the end coating. In other embodiments, a conductor may beapplied against the end coating. The conductor may be formed of multipleconductive strands. The strands may be formed to distribute mechanicaland electrical loads substantially evenly across the entire terminationzone. The size of the termination zone area may be selected to providean acceptable current density such that thermal and mechanical loads donot become excessive at any one location. In one embodiment, theconductor may be a braided strap. A clamp may urged the conductoragainst the end coating, resistive coating, or some other interfacelayer applied to the substrate. The clamp may maintain the conductoragainst the underlying surface, while accommodating expansion withtemperature, without harming mechanical bonds between the resistivecoating and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the present inventionwill become more fully apparent from the following description andappended claims, taken in conjunction with the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are, therefore, not to be considered limiting of itsscope, the invention will be described with additional specificity anddetail through use of the accompanying drawings in which:

FIG. 1 is a side elevation view of a heater unit in accordance with theinvention;

FIG. 2 is a front elevation view of a heater assembly including multipleunits of the apparatus illustrated in FIG. 1;

FIG. 3 is a perspective view of one embodiment of a coated conduit inaccordance with the invention;

FIG. 4 is a schematic, side, elevation, cross-section view of a portionof the apparatus of FIG. 3, illustrating the comparative positions ofthe substrate, resistive coating, end plating (coating), and connectionscheme for introducing electricity to the apparatus;

FIG. 5 is a block diagram of one embodiment of a process for making aheating unit in accordance with the invention;

FIG. 6 is a graph illustrating a relationship between a bath time in aplating composition, illustrating the effect of normalized resistanceper square in ohm-inches per inch;

FIG. 7 is a graph illustrating a comparison between terminatedresistance and watt density in a heater in accordance with the inventionas a function of the cured resistance of a coating in accordance withthe invention, further illustrating typical termination resistanceadjustment depending upon the cured resistance of a conductive andresistive coating;

FIG. 8 is a chart illustrating a change in heating area (function oftermination distance), in order to correct for variations in cured (heattreated) resistance values in a resistive coating of an apparatus inaccordance with the invention;

FIG. 9 is a side elevation of a termination in accordance with thepresent invention;

FIG. 10 is section view of the termination illustrated in FIG. 9;

FIG. 11 is a side elevation of an alternative embodiment of atermination in accordance with the present invention;

FIG. 12 is plan view of an embodiment of a termination conductor inaccordance with the present invention; and

FIG. 13 is a perspective view of a termination in accordance with thepresent invention as applied to a conduit for heating fluids passingtherethrough.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be readily understood that the components of the presentinvention, as generally described and illustrated in the Figures herein,could be arranged and designed in a wide variety of differentconfigurations. Thus, the following more detailed description of theembodiments of the system and method of the present invention, asrepresented in the FIGS. 1 through 11, is not intended to limit thescope of the invention, as claimed, but is merely representative of thepresently preferred embodiments of the invention.

The presently preferred embodiments of the invention will be bestunderstood by reference to the drawings, wherein like parts aredesignated by like numerals throughout. Those of ordinary skill in theart will, of course, appreciate that various modifications to thedetailed schematic diagram may easily be made without departing from theessential characteristics of the invention, as described in connectionwith the Figures. Thus, the following description of the Figures isintended only by way of example, and simply illustrates certainpresently preferred embodiments consistent with the invention as claimedherein.

Referring to FIGS. 1-3, an apparatus 10 may be created for heating orotherwise handling process fluids such as those used in thesemiconductor industry. The semiconductor-processing industry requiresultra-pure, de-ionized (DI) water, acids, and the like. A conduit 12 maybe formed of a comparatively rigid material such as quartz.

Fused quartz resists distortion due to changes in temperature and time,providing dimensional stability and repeatable structural properties.Additionally, quartz is substantially non-reactive with processingfluids and meets industry parts-per-billion (or even trillion) purityrequirements in acids and water, such as de-ionized water.

Fittings 14, 16 may support the conduit 12 and apply force 18 from apressure plate 32, loader (e.g., spring) 34, baseplate 36 and adjuster38 to support a suitable seal 20. An inlet 22 and outlet 24 may conveyfluid along the length 45 of the apparatus 10 from a manifold 46. Aplurality of the individual apparatus 10 may be assembled as a heater 47in a cabinet 48 or outer frame 48 enclosing an outer envelope 49.

The heater 47 does not expose metals to the process fluid inside theconduits 12. In one presently preferred embodiment, a resistive coatingon the conduit 12 heats the conduit 12. The heat passes through the wallof the conduit 12 into the process fluid therein.

Referring to FIG. 3, a conduit 12 may be formed of a crystallinematerial such as fused quartz. In general, a conduit 12 may be of anysuitable shape. For example, a flat plate may be fitted, as a window, orthe like, against a structure suitable for sealing the window. A coatingmay be applied to such a substrate. Accordingly, the term conduit 12,may include any substrate, of any shape, suitable for receiving acoating for generating electrical resistance heating.

The conduit 12 may define an axial direction 50 a and radial directions50 b. A wall 52 of the conduit 12 may extend in an axial direction 50 aand circumferentially 50 c. The wall 52 may define, or be defined by, anouter surface 54 and an inner surface 56.

In selected embodiments, an outer surface 54 may be treated, such as bymechanical etching, to provide a portion of roughened surface 58. Thetextured surface 58 may be prepared by a mechanical abrasive action,such as grit blasting, bead blasting, or sandblasting. Accordingly, in acrystalline material, such as quartz, small crystalline chunks may beremoved from the surface 54, leaving small, angular, crystallineinclusions in the surface 54.

The techniques and materials used in the preparation and coating of theouter surface 54 may be used to coat an inner surface 56. For example,the wall 52 may be treated to provide a textured surface on the innersurface 56. Concentric conduits 12 maybe employed to provide additionalheating. In such an embodiment, the inside surface 56 of the innerconduit 12 may be provided with a heater 10 while the outside surface 54of the outer conduit 12 is provided with another heater 10. The fluidmay then be heated at both the inner flow and outer flow extremeswithout being exposed to any potential contamination.

The coating 60 may typically be a substantially continuous film 60extending over the area of the substrate to which the heat is to beapplied. In a heating conduit 12 embodiment, the coating 60 may extendaxially 50 a and circumferentially 50 c about the outer surface 54. Anend coating 62, applied over the basic coating 60, may be formed of thesame material, a similar material, or a material having differentmechanical properties. The end coating 62 may be of any suitablematerial selected to maintain mechanical integrity and adherence betweenthe coating 60 and the textured surface 58. In certain embodiments, theend coating 62 may be applied by a method other than depositing orplating. In alternative embodiments, the end coating 62 may simply beadditional material, identical to the coating 60. The end coating 62 maydecrease the resistance of the coating 60 by providing increasedcross-sectional area along a portion of the length. Thus, the endcoating 62 effectively shortens the resistive coating 60.

The end coating 62 may provide less resistance along a given direction50 a, 50 c than the resistive coating 60. That is, the end coating 62may include more material per unit of area in order to distributeelectricity from a connector lug 64 in an axial 50 a and acircumferential direction 50 c. Thus, the end coating 62 becomes adistributor or a manifold for electricity provided to a lug 64 orconnector 64 suitable for receiving a wire delivering current to theresistive coating 60.

A protective coating 66 of a suitable, conformal material may be appliedto reduce scratching, wear, and chemical reaction of the resistivecoating 60, thus extending the operational lifetime thereof. The appliedcoatings 60,62, 66 need not extend from end 68 to end 70 of thesubstrate 12. A distance 72 of smooth surface 54 may remain in order tosupport sealing of the ends 68, 70 as described herein. Smooth, fired,quartz formed in a lip 30 may provide sealing, strength, manufacturing,and handling advantages.

A lug 64 or band 64 may serve as a base 64 for a connection 65 forelectrical power inputs. The lug 64 may be spaced a selected distance 74from either end 68,70 of the conduit 12. A end coating 62 of conductivematerial may distribute electricity to the resistive coating 60. The endcoating 62 may be placed at any suitable location along the length ofthe of the conduit 12.

Electricity travels between the bands 64 and end coatings 62 along aresistance length 76. Power dissipation for heating requires current andresistance. The resistivity and conductivity of the coating 60 may beselected and balanced to generate a desired wattage dissipation per unitarea. Accordingly, the resistivity and conductivity of the coating 60may be controlled by selecting coating 60 thickness and length 76.

The coating 60 may be designed and applied within parameters engineeredto balance several factors. For example, if the textured surface 58 istoo rough, the conduit 12 may fail under test pressures. Ifinsufficiently rough, the textured surface 58 may provide inadequateadhesion forces between the resistive coating 60 and the outer surface54 of the conduit 12 or substrate 12.

The resistive coating 60 may be benefitted from uniformity ofconductivity and cross-sectional area along the length 76 in an axialdirection 50 a. An excess of the coating 60 may promote unitary motionthereof. With the application of thermal and mechanical loads, theunitary motion of the resistive coating 60 may mechanically separate theresistive coating from the textured surface 58. This may be particularlyevident when dealing with material having different coefficients ofthermal expansion. Ceramics and other materials, such as quartz, havevery low coefficients of thermal expansion. In contrast, most metalsprovide substantial expansion with increased temperature. Accordingly,at elevated temperatures, the coating 60 tends to expand and separate asa continuous annulus surrounding the conduit 12.

At a microscopic level, the coating 60 tends to shear away from themicroscopic inclusions developed in the textured surface 58. Thus, abalance in application of the coating 60 is required to balance theforces due to thermal expansion with the mechanical bond between thecoating 60 and the inclusions in the textured surface 58.

The effective resistance of the coating 60 changes as the coating 60 isheat treated. Heat treatment does not melt the deposited coating 60.Nevertheless, metallurgical grain boundaries form, grow, and affectelectrical conductivity in the coating 60. If the effective resistanceis too high, the heater 10 may not provide sufficient energy inputthrough the wall 52 into a fluid flow 78. If the resistance is too low,the heater 10 may provide an output outside the desired range ofcontrol. In some apparatus, excessive heating may damage equipment,including fracturing solids as a result of differentials in expansion.

The end coating 62 or band 62, if applied too thickly, may overcome theadhesion or other bonding between the end coating 62 and the resistivecoating 60. Alternatively, the end coating 62 may maintain a sufficientbond with the coating 60, but separate the coating 60 from the texturedsurface 58. This is particularly common if either the resistive coating60, end coating 62, or their combination is too thick and mechanicallyrigid. Similarly, as with the resistive coating 60, applying the endcoating 62 too thinly, tends to reduce the average number of atoms atany site, yielding poor uniformity, and inadequate process control forreliable current conduction.

Excessive resistance in the end coating 62 may generate too much heat.Excessive heat may destroy the connection between the end coating 62 andthe resistive coating 60, or separate both from the textured surface 58.

A lug 64 or connector band 64 may be secured with the sameconsiderations required for the coatings 60, 62. Namely, excess materialmay provide excessive strength and generate unitary motion.Additionally, insufficient material may create hot spots. The lug 64 orconnector band 64 materials may be selected to provide flexibility,malleability, elasticity, and plasticity.

Referring to FIG. 4, a wall 52 may be thought of as a substrate 80.Thus, a substrate 80 may generalize a conduit 12 into any particularshape, open, closed, and so forth. As discussed, a thickness 82 of asubstrate 80 provides mechanical integrity and strength in a conduit 12.In use, the conduits 12 may have internal pressure loads appliedthereto. Excessive thickness 82 may generate a stress differentialbetween the inner and outer surfaces 56, 54. Additionally, the thickness82 may be affected by the inclusions in the textured surface 58. Thethickness 82 may benefit from being sufficiently large in comparison tothe inclusions of the textured surface 58, thus mitigating the risk ofcrack propagation

The thickness 73 of the resistive coating 60 may be preciselycontrolled. The thickness 73 may be on the order of numbers of atoms upto a few millionths of an inch. In selected embodiments, the thickness73 is selected to be within an order of magnitude of the size ofinclusions in the textured surface 58. In an alternative embodiment, thethickness 73 may be selected to be more than an order of magnitudesmaller than the size of inclusions in the textured surface 58.Accordingly, the coating 60 may appear like a crepe material. This crepemay be a thin, crinkly film following the peaks and valleys of theinclusions formed in the textured surface 58.

Thermal expansion due to a rise in temperature may be accommodated bylocalized bending of portions of the coating 60. If the thickness 73becomes too great, however, the coating 60 behaves as a beam extendingin the circumferential direction 50 c and the axial direction 50 a.Accordingly, the beam may change diameter, applying comparatively largeradial forces withdrawing the small irregularities from their placesfilling the inclusions in the textured surface 58.

Excellent thermal contact between the coating 60 and the conduit 12requires superior adhesion by selecting an appropriate thickness 73. Thethickness 73 may be successfully selected to provide mechanicalcompliance with the textured surface 58 while providing uniformity.Thus, the selection of the resistive material 60, thickness 73, andsubstrate thickness 82 may be used to control heat input for a fluidflow 78 while maintaining mechanical integrity and thermal conductivity.

A interface layer 63 may be selected from a softer material than thecoating 60. Selecting an interface layer 63 material that iscomparatively malleable and thin, while having comparatively higherelectrical conductivity than the coating 60, may produce suitablemechanical and electrical integrity.

A roughness level or inclusion height 90 may be detected by thereflection of light or sheen of the roughened surface 58. The roughnessheight 90 dramatically affects the sheen of the roughened surface 58,even with comparatively minimal roughness 90. Thus, the adequacy of theroughness height 90 may be detected as well as gauged by a visualinspection.

Excessive roughness height 90 may result from removing too much of thewall 52 from the textured surface 58. Controlling grit size (e.g. beadsize) and time of application may provide a suitable roughness height90. The roughness height 90 should accommodate mechanical lodgment ofatoms of the coating 60 within inclusions in the surface. Thus,micro-mechanical anchors grip the thin coating 60 an maintain it againstthe outer surface 54.

The quality of the roughness height 90 may be additionally be gauged bythe crystalline sharpness and angularity of the inclusions. Spalling ofsubstrate 12 material from the outer surface 54 under the influence ofgrit, bead, sand blasting, or the like may tend to break the substratealong crystal boundaries. In this manner a fully randomized set ofinclusions, including concavities overhung by sharp crystalline corners,may be provided. Such inclusions may securely capture pockets of atomsof the coating 60.

The resistive path of the coating 60 may be affected by the roughnessheight 90. A smooth outer surface 54 tends to provide a direct currentpath. A textured surface 58, provides an indirect path over hills andvalleys of the inclusions formed in the textured surface 58. Thus,providing too great a thickness 73 of the resistive coating may decreaseresistivity reducing the heating dissipation below a desired value.

Referring to FIG. 5, a method for manufacturing the heater 10 inaccordance with the present invention may include providing 102 theconduit 12 or other substrate 80, followed by suitable masking 104 andtexturing 106. Texturing 106 may include bead blasting, sand blasting,grit blasting, or etching by other means. In selected embodiments, beadblasting may provided considerable uniformity in the fracture mechanicsof forming inclusions in a substrate without sacrificing mechanicalintegrity thereof. The texturing 106 is may provide mechanical grip, asdiscussed hereinabove. The roughness height 90 may be selected to createinclusions that will not compromise the mechanical integrity of theconduit 12.

The wall thickness 82 may be selected to balance heat transfer andstructural advantageous. Thermal gradients may be considered in view ofthe substrate thickness 82 and thermal stresses created by changingtemperatures of the apparatus 10.

A thin film 60 is applied in a plating process 108. In one embodiment,electroless nickel plating forms a suitable resistive coating 60. Theplating process may be continued for a time selected to provide adesired thickness 73. The thickness 73 of the resistive coating 60 maybe selected to balance current-carrying capacity of the coating 60,mechanical stiffness and strength limits required to maintain adhesion,and coating uniformity. In certain embodiments, balancing involvesadjusting resistive coating thickness 73 to achieve uniformity ofperformance, either mechanical, thermal, electrical, or a combinationthereof.

The plating process 108 may be selected from the group consisting ofvapor deposition, sputtering, painting, sintering, powder coating, andelectroless plating. In electroless plating, such as electroless nickelplating, application 109 of a surfactant may greatly improve the qualityof the coating 60. Application 109 of a surfactant may involve asurfactant scrub 109 in which vigorous application of force breaks downany pockets of gas that might adhere to concavities in the texturedsurface 58. Thereafter, the coating 60 may form, maintaining acontinuous mechanical structure about the inclusions of the texturedsurface 58.

After the resistive coating 60 has been applied 108, it may beadvantageous to heat treat 110 the substrate 12 and coating 60. In oneembodiment, the heat-treating process 110 involves a metallurgical heattreatment 110. Such a process 110 does not elevate temperaturessufficiently to melt the metallic coating 60. Rather, temperatures areelevated, raising the energy level of various atoms within the coating60, to encourage migration of interstitial materials. Migration ofinterstitial materials may foster growth of various grain boundaries.Growth of grain boundaries affects the binding of electrons intoorbitals of various atomic or molecular structures. Thus, theheat-treating process 110 may substantially affect electricalconductivity. Accordingly, the time and temperature of the heattreatment process 110 may provide a control over the effectiveelectrical resistivity of the coating 60.

In certain embodiments, heat treating 110 may include a surfacetreatment. In one embodiment, an application 111 or deposition 111 (e.g.vapor deposition) of a surface-protecting layer may include adding acomposition (e.g., a silicate) to the heat-treatment environment. Theapplication process 111 may include masking portions of the coating 60that may later be coated with additional conductive materials. Theprotective process 111 provides a non-reactive coating or passivatingcoating to reduce oxidation of the resistive coating 60 during heattreating 110.

Following the heat-treating process 110, a termination process 112provides end coatings 62. The placement of the termination may beinfluenced by a determination of the electrical length 113 needed toprovide appropriate heating. In certain embodiments, the terminationprocess 112 may include application 114 of a termination coating 62 orend coating 62 to reduce the resistance of the heater 10. Resistance maybe lowered by half an order of magnitude. The thickness 77 of the endcoating 62 must be balanced to provide good current distribution,without compromising the mechanical integrity of the bond between theconductive-resistive materials and the conduit 12 or substrate 80.

In selected embodiments, the termination process 112 may involveapplication 114 of a end coating 62 having a specific length 75calculated to provide a precise power delivery in the heater 10.Similarly, a soft, compliant, conductive material may be added 116 overa portion of the end coating to form an interface layer 63 for receivinga connector 65. The connector 65 may be any suitable electricalconnection. In one embodiment, the connector 65 is an electrical lead 65electrically secured to the interface layer 63 or some other underlyinglayer (e.g. end coating 62, conductive coating 60). In an alternativeembodiment, the band 64 may be formed to transfer electricity to theconductive coating 60. In such an embodiment, a braid 64 may be applied118. After application of the braid 64, a clamping mechanism 67 may beapplied. The clamp 67 may be adjusted (e.g. tightened) to apply aclamping pressure 120. The clamping pressure may urge the braid 64against the underlying layers. A protective, conformal coating 66 may beapplied 122 following, or as part of, the termination process 112.

Referring to FIG. 6, a graph 130 having a time axis 132 and resistanceaxis 134 illustrates various experimentally derived data points 136. Thevalues 136 characterize the effect of time, during plating, on theinitial resistance 134 of the coating 60. The scales are logarithmic.Thus, the process results in resistance being dependent upon a power oftime. The relationship does not appear to change dramatically at anypoint on the graph 130.

Referring to FIG. 7, a graph 140 of a resistance in a range 142corresponds to a value of heat-treat temperature in a domain 144 oftemperatures for the coating 60. The values 148 reflect the adjustmentof resistance in ohm-inches per inch, due to a particular temperatureduring heat treating of the coating 60. The resistance of the coating 60may vary due to variations in controlled parameters, such as the timeand temperature associated with heat treatment. Parametric controls mayvary during the plating process, and the heat-treating process 110.Thus, FIG. 7 reflects an ability to adjust the effective resistance ofthe apparatus 10 according to the heat-treat temperature.

Referring to FIG. 8, a graph 150 shows both a percentage 152 ofavailable surface area heated by the coating 60 and a watt density 154as a function of resistance per square 156. The graph 150 shows thecorrection ability for any given resistivity resulting from theheat-treat process 110. That is, given a particular value of the curedresistance 156, a final percentage 152 of area to be heated (powered)may be determined. Thus, the exact locations of the end coatings may bedesigned to obtain the desired heated area. Similarly, for a particularcured resistance 156, a watt density 154 may be determined. Theseresults illustrate the influence that the end termination process 112can have on correcting the overall value of resistance of the coating 60in an apparatus 10.

Referring to FIGS. 9 and 10, as discussed hereinabove, a balance existsbetween the ability of the resistive coating 60 to provide the properheat dissipation and the ability to maintain mechanical adherence to thesubstrate 80. As a result, it may be advantageous to have a termination158 that does not interfere with the mechanical and electrical integrityof the underlying coatings (e.g. resistive coating 60, end coating 63,or interface layer 63) during fabrication or operation.

A termination 158 may distribute mechanical and electrical loads so thatload densities are substantially evenly distributed and withinacceptable limits and tolerances. Mechanical loads may include allforces, such as shear, tensile, compression, expansion, contraction, andthe like, that may be imposed on or by a termination 158. Electricalloads may include voltage differentials, current densities, and thelike. Electrical loads and the heating that may accompany them, oftencause material expansion and give rise to the mechanical loads.Acceptable tolerances may be defined as a level of mechanical andelectrical loading that provides an acceptable termination. Thetolerance levels may include a safety factor to provide a more reliableresult.

FIGS. 9 and 10 illustrate an embodiment of a termination 158 that mayprovide the desired mechanical and electrical load distribution. Such atermination 158 may cooperate with a substrate 160. The substrate may bea material selected to meet desired chemical inactivity, heat transfer,strength, rigidity, durability, electrical, mechanical, adhesion, orthermal expansion characteristics. In selected embodiments, thesubstrate 160 is fused quartz.

The substrate 160 may be prepared to receive a conductive coating 162.As discussed hereinabove, the substrate 160 may be prepared by amechanical abrasive action, such as grit blasting, bead blasting,sandblasting, or a similar process. The conductive coating 162 may beapplied by a suitable method such as plating, depositing, vapordeposition, sputtering, painting, sintering, powder coating, electrolessplating, or the like. A suitable material may be chosen as theconductive coating 162. The material may be selected to provide thedesired electrical resistivity, electrical conductivity, mechanicalstrength, adherence to the substrate, or durability. In certainembodiments, the conductive coating 162 comprises nickel applied by anelectroless plating process. In other embodiments, other metals, such asgold, silver, copper, etc., having suitable resistance may be used atsuitable thicknesses. In selected applications and embodiments, it maybe beneficial to provide an interface layer 164 to extend over the areato which the termination 158 is to be applied. The interface layer 164may provide a selectively deformable layer to receive a conductor 166. Aclamp 168 may apply a mechanical load 169 to the conductor 166 to ensurean effective electrical contact between the conductor 166 and theunderlying surface (e.g. interface layer 164). A lead 170 in intimatecontact may deliver an electrical load to the conductor at an attachmentpoint 172.

The conductor 166 may be formed to provide mechanical load distribution.For example, the conductor 166 may be formed of multiple strands 174.The strands 174 may be crimped, bent, twisted, woven, or otherwiseformed to produce multiple points of contact between themselves and theclamp 168 and/or between themselves and the underlying surface (e.g.interface layer 164). Moreover, formation processes (e.g. crimping,weaving, twisting, etc.) of the strands 174 may effectively createmultiple deflectable springs 176. In the illustrated embodiment, thestrands 174 are woven to effectively form leaf springs 176 (fibers 176).In such a configuration, a strand 174 a and the leaf spring 176 formedtherein, may distribute a mechanical load 169 applied by a clamp 168 tocreate at least two smaller loads 178. In a similar manner, the smallerloads 178 may be distributed, by contact between interleaving fibers 176(leaf springs 176), thus further propagating the applied load 169 toother locations.

As previously discussed, electrical loads and the heating that mayaccompany them, often cause thermal expansion of material and give riseto substantial mechanical loads. In many applications, where materialsin intimate contact have different coefficients of thermal expansion,this expansion may range from undesirable to catastrophic. For example,an expanding conductor 166 may apply excessive compressive hoop stressesto the conductive coating 162, causing it to separate radially from thelower-expanding or non-expanding substrate 160. Additionally, expansionof the conductor 166 may cause uneven distribution of electrical loads,resulting in hot spots. Hot spots are undesirable for many reasons,including variations in conductivity, electrical overheating, burnout,mechanical distortions and de-lamination, or failure of the termination158.

The conductor 166 may be formed to distribute thermal expansion, or evenredirect it, thus limiting net movement between the conductor 166 andany adjacent material (e.g. interface layer 164, clamp 168). Forexample, the conductor 166 maybe formed of multiple strands 174. Thestrands 174 may be crimped, bent, twisted, woven, or otherwise formed toproduce multiple tortuous paths. The tortuous paths of the strands 174may create multiple deflectable springs 176 (e.g. leaf springs 176).Upon expansion or contraction of the material of the strands 174, thesprings 176 may deflect to absorb the displacement motion induced by thechange in physical size. The result may be a substantially limited netexpansion of the conductor 166 with respect to its surroundings. Thisembodiment may be particularly suited for terminations 158 involvingseveral materials with differing coefficients of thermal expansion.

As discussed hereinabove, it may be beneficial to have an interfacelayer 164. The interface layer 164 may be formed of a suitable materialselected to provide a desired combination of adherence, elasticity,plasticity, resistance, and conductivity. The material of the interfacelayer 164 may be selected to adhere to an underlying coating (e.g.conductive coating 162) without damaging the coating or causing theseparation thereof during thermal cycling. The interface layer 164 mayalso provide a balance of elasticity and plasticity. This balance maysupport effective electrical contact between the interface layer 164 andthe conductor 166. In selected embodiments, the interface layer 164 maybe a comparatively thin deposit of solder 164, providing substantiallyno effective rigidity to the underlying conductive coating 162.

In certain embodiments, the interface layer 164 may elastically deflectand plastically yield locally around contact points 180. As a load 169is applied, the conductor 166 may embed itself into the interface layer164 a distance effective to provide increased electrical contact areatherebetween. Displaced interface material 182 may form around eachfiber 176 (spring 176) increasing the contact area 184 about theprincipal contact point 180 or contact region 180. Larger contact areas184 promote lower local electrical resistance and, therefore, decreasedheat generation. As discussed, decreased heat generation may reducethermal expansion and the risk of overheating. The elasticity of theinterface layer 164, as well as the lateral bends and springiness of theconductor 166 (fibers 176) may combine to maintain effectively constantelectrical contact throughout thermal cycling of operational use. Insuch a manner, mechanical and electrical loads may be distributed toresist overheating, separation, de-lamination, or other forms offailure.

The conductor 166 may be made of multiple strands 174. The strands 174may be formed to move, expand, shift, or otherwise repositionsubstantially independently from one another. That is, movement of onestrand 174 a does not necessarily require the movement of a neighboringstrand 174 b. A conductor 166 in accordance with the present inventionmay be formed from one or more strands 174. The strands 174 may beformed of a suitable material having the desired conductivity,elasticity, malleability or formability, and durability. The conductor166 may be coated with a material selected to discourage bonding,galling, or sticking thereof to a surrounding surface (i.e. surfaceswith which the conductor 166 is in contact with). Silver may operate toimprove conductivity and resist galling. In one embodiment, theconductor 166 is a braided strap 166 made of copper strands 174 coatedwith silver to reduce adherence to an interface layer 164 of solder.

Other films, layers, coatings, or the like may intersperse between theconductive coating 162, interface layer 164, and conductor 166. Thesecoatings (e.g. end coatings 62) may adjust the resistivity of theconductive coating 162, enhance adherence, reduce separation, increasedurability, or otherwise enhance the operation of the apparatus 10. Theelements of a termination 158 in accordance with the present inventionmay be applied in conjunction with these other films.

Referring to FIG. 11, in an alternative embodiment, the interface layer164 may be omitted. The conductor 166 may include an electricallyconductive interior 186 and a compliant exterior 188. The conductiveinterior 186 may provide the mechanical resilience for theload-distributing spring effect described hereinabove. As a load 169 isapplied, the compliant exterior 188 may deform to match the surfaceagainst which it is being pressed. Displaced exterior material 182 mayincrease the contact area 184 of the contact points 180. Referring toFIG. 12, a conductor 166 in accordance with the present invention may beformed to distribute electrical loads. As discussed hereinabove, hotspots are undesirable because they may result in electrical overheating,burnout, or other failure modes of the termination 158. Distributingelectrical loads may greatly reduce the occurrence of hot spots. Inselected embodiments, electrical load distribution may be accomplishedby a woven or braided conductor 166.

A braided conductor 166 may be made of several conductive strands 174.Each strand 174 may conduct only a fraction of the electrical current ofthe whole termination 158. As a result, a contact point 180 a of reducedor increased electrical resistance on a strand 174 a likely will notdraw a large portion of the total current applied to the conductor 166nor be allowed to develop a voltage drop likely to support an arc.Additionally, the decreased resistance of a parallel electron path froma neighboring strand 174 b to strand 174 a may compensate for thevariation in resistance of the contact point 180 a thus, reducing thelikelihood that an electron will find any path of significantly higheror lower resistance through a neighbor of any contact point 180 a.

When a contact point 180 a is not actually in contact with theunderlying surface (e.g. interface layer 164), electrons may be impartedto the underlying surface at the many neighboring contact points 180 b,180 c, 180 d, and 180 e maintaining low resistance and low voltagedrops. In this manner, the occurrence of cold spots, areas of less thanaverage current, or gaps subject to arc, may be reduced.

FIG. 13 illustrates one selected embodiment of a termination 158 inaccordance with the present invention. A substrate 160 may be formedinto a cylindrical conduit 190. The substrate 160 may be prepared andthen coated with a conductive coating 162 for providing a pre-determinedbalance of resistance and current flow. An interface layer 164 may beplaced over the conductive coating 162 in the termination zone 192. Inthe illustrated embodiment, the termination zone 192 is a circularcontinuous band. A conductor 166 (e.g. a braided strap) may be placeddirectly against the termination zone 192, thus, encircling the conduit190. A lead 170 may conductively secure (e.g. by solder or othermechanical joint) to the conductor 166 at an attachment point 172. Aclamp 168 may circumferentially encircle the conductor 166 and maintaina contact force of each strand 174 against the interface layer 164 in adirection normal to the surface. The clamp 168 may be a comparativelystrong clamp 168 circumferentially configured to flex enough to equalizeradial stresses. In selected embodiments, the conductor 166 may bescored or otherwise shaped to create a channel 194 or circumferentialindentation 194 to facilitate rapid alignment and assembly of the clamp168.

From the above discussion, it will be appreciated that the presentinvention provides apparatus and methods for heating ultra pure fluidsin a hyper-clean environment. Power densities are very high, whileheater reliability is superior. Meanwhile, manufacturing is rapid yetreliable, and adjustments are available to produce high yields of highlypredictable product.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrative,and not restrictive. The scope of the invention is, therefore, indicatedby the appended claims, rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. An apparatus for operably connecting an electrical sourceto a conductive coating, the apparatus comprising: a substratecomprising a structural material; a conductive coating applied to thesubstrate; a conductor comprising strands configured to be separable andelectrically conductive for providing electricity to the conductivecoating; a clamping mechanism configured to apply a clamping load urgingthe conductor toward the conductive coating; and the conductor, whereinthe strands are further configured to distribute mechanical stress andstrain induced by thermal expansion and the clamping load sufficientlyto substantially reduce damage to the mechanical and electricalintegrity of the conductive coating.
 2. The apparatus of claim 1 whereinthe strands are configured to extend along tortuous paths providingredistribution of electrical and mechanical loading directed toward theconductive coating.
 3. The apparatus of claim 2, wherein the strands areconfigured to provide spring motion and corresponding forces fordistributing mechanical loads between the clamping mechanism and theconductive coating.
 4. The apparatus of claim 2, wherein the strands areconfigured to provide distribution of mechanical motion of the conductorin order to minimize shearing forces directed to the conductive coating.5. The apparatus of claim 1, further comprising an interface layerpositioned between the conductor and the conductive coating.
 6. Theapparatus of claim 5, wherein the strands are unbonded to one anotherand to the interface layer.
 7. The apparatus of claim 6, wherein theconductive coating, the interface layer, and the conductor providedissimilar metals at each contact point therebetween.
 8. The apparatusof claim 1, wherein the substrate comprises a material having acoefficient of thermal expansion substantially different from that ofthe conductive coating.
 9. The apparatus of claim 8, wherein thesubstrate comprises quartz.
 10. The apparatus of claim 1, wherein thestrands are configured to provide parallel paths for distributingcurrent and reducing voltages with respect to the conductive coating.11. An apparatus for operably connecting an electrical source to aconductive coating, the apparatus comprising: a substrate comprising astructural material; a conductive coating applied to the substrate; aninterface layer applied over the conductive coating and configured toconduct electricity thereto while transferring insufficient force toseparate the conductive coating from the substrate; and a conductor forproviding electricity to the interface layer, the conductor beingpositioned in contact with the interface layer and comprising strandsconfigured to be separable and electrically conductive.
 12. Theapparatus of claim 11, wherein the interface layer is further configuredto selectively distort elastically and plastically in response tolocalized loading of the strands thereagainst.
 13. The apparatus ofclaim 12, further comprising a clamp configured to impose a load urgingthe strands against the interface layer.
 14. The apparatus of claim 12,further comprising a clamp configured to impose a load configured toselectively distort the interface layer to receive the strands therein.15. The apparatus of claim 14, wherein the interface layer is configuredto elastically distort to an extent selected to maintain an effectivecontact pressure between the interface layer and the strands.
 16. Theapparatus of claim 15, wherein the interface layer is configured toplastically distort to an extent selected to provide an effectivecontact area between the interface layer and the strands.
 17. Theapparatus of claim 11, wherein the conductive coating has a coatingthickness, and wherein the strands have a strand thickness substantiallywithin an order of magnitude of the coating thickness.
 18. The apparatusof claim 11, wherein the substrate is formed of a crystalline material.19. The apparatus of claim 18, wherein the crystalline material isquartz.
 20. The apparatus of claim 11, wherein the conductive coatingcomprises nickel.
 21. The apparatus of claim 11, wherein the interfacelayer comprises a solder adhered to the conductive coating andnon-adhered to the strands.
 22. The apparatus of claim 11, wherein astrand of the strands further comprises a copper core covered with asilver coating.
 23. The apparatus of claim 11, wherein a strand of thestrands has mechanical properties providing elasticity selected toprovide a spring force between the clamp and the interface layer. 24.The apparatus of claim 1, wherein the strands are braided.
 25. Theapparatus of claim 11, wherein the substrate is roughened to receive theconductive coating, and configured to have a roughness height selectedto balance a value of heat transfer through the substrate, mechanicalintegrity of the substrate, and adhesion of the conductive coating tothe substrate.
 26. The apparatus of claim 11, wherein the conductivecoating is deposited at a thickness characteristic of a process selectedfrom spraying, sintering, flame spraying, vapor deposition, and plating.27. The apparatus of claim 11, wherein the substrate is a tubular memberhaving first and second ends and configured to heat fluids therein, theconductive coating is a heating element further comprising a terminationzone proximate at least one of the first and second ends, thetermination zone comprising a region of reduced electrical resistancefor distributing electrical current to the conductive coating.
 28. Anapparatus for operably connecting an electrical source to a conductivecoating, the apparatus comprising: a substrate comprising a structuralmaterial; a conductive coating applied to the substrate; an interfacelayer applied over the conductive coating and configured to transfer tothe conductive coating insufficient thermally-induced force to damagethe integrity of the conductive coating with respect to the substrate;and a conductor for providing electricity to the interface layer, theconductor being positioned in contact with the interface layer andcomprising strands configured to be separable and electricallyconductive.
 29. A method for connecting an electrical lead to acomparatively thin coating, the method comprising: providing a substratehaving a thickness; applying to the substrate a coating, comparativelythin and comprising a conductive material; positioning a conductor,comprising a plurality of independent strands, opposite the substrate ina position to provide electricity to the coating; applying a clampingload urging the conductor toward the coating; and redistributing, by theindependent strands, mechanical and electrical loads directed toward thecoating.